34 THE ARMY HORSE IN ACCIDENT AND DISEASE. 



color. The dark impure blood is then collected through the small 

 veins into the larger veins and thence into the right auricle from 

 which it started. This round or circuit, which is constantly going 

 on, gives rise to the name circulation. 



It has been stated that arteries convey the blood away from 

 the heart and that veins return it. In supplying the body, arteries 

 carry pure blood and veins carry impure blood. When, however, 

 the impure blood is sent to the lungs for purification, it is conveyed 

 in an artery and the pure blood returns in a vein. These two important 

 exceptions must be carefully noted. 



Arterial ramijication. 



The large artery given off from the l^ft ventricle of the heart 

 is the common aorta, which passes upward and forward for 2 or 3 

 inches and divides into the anterior aorta and the posterior aorta, 

 supplying, respectively, the fore and hind portions of the body. 



The anterior aorta is very short (1 or 2 inches), passes upward and 

 forward under the trachea and between the lungs, and divides into 

 the right and left brachial arteries, each supplying blood to one of the 

 fore limbs and its neighboring muscles. 



The right brachial artery gives off a large branch, called the com- 

 mon carotid. There is no corresponding branch of the left brachial. 

 The common carotid is short and immediately divides into two 

 branches, right and left carotids, which pass up the neck, at first 

 under the trachea and then on either side; they follow the trachea 

 to the throat, where they divide into branches, supplying the head. 



The brachial arteries continue toward the front part of the thorax, 

 winding around the first ribs, and divide into branches, supplying the 

 fore limbs. The main branch of each is here named the humeral 

 artery. 



The humeral artery descends along the inner side of the humeiois, 

 and just above the elbow joint divides into the anterior and posterior 

 radial arteries. 



The anterior radial descends over the front surface of the elbow 

 joint, passes down in front of the radius, and approaches the knee 

 below the extensor pedis muscle, where it divides into numerous 

 branches, supplying blood to the surrounding tissues. The posterior 

 radial is a continuation of the humeral artery, passing down the inner 

 side of the fore leg, inclining back and dividing at the lower end of 

 the radius into the large and small metacarpal arteries. 



The small metacarpal passes outward from the inner and back i)art 

 of the knee and, running downward, supplies nourishment to the 

 surrounding tissues. The large metacarpal is a continuation of the 

 posterior radial. It runs down the back of the knee, in company 

 with the flexor tendon; above the fetlock it passes between the 



